On Wed, Jan 29, 2014 at 11:07 AM, MCLAREN, Donald
<[log in to unmask]> wrote:
Thanks, Donald and Jonathan, for your responses.
> In the between-subject approach, you'd only include 1 condition and compare
> the pre- and post.
Could you expand on this a bit? Why wouldn't I include the other three
conditions and nuisance regressors in my model? Is it actively
detrimental, or does it just not matter since I'm only intending to
compare the parameters for pre- and post?
>I think for what you are interested in doing (as you may
> become interested in the connectivity changes relative to the other
> conditions), I would probably use the FSL implementation of the gPPI
> approach (I'd say SPM, but this is the FSL list). "I'd consider creating a
> model that is identical to the standard block analysis of your data (you say
> you have 4 conditions, so 4 task regressors), then add the PHYS timecourse
> you're interested in (just as prescribed in Jill's tutorial), and then
> create a PPI (Feat interaction term) for each of the 4 conditions X PHYS
> regressor. You'll have 9 regressors in the model (4 conditions, 1 PHYS, and
> 4 PPIs: 1 per condition)." -JH. You can repeat this for the two time periods
> and then make contrasts for pre-post or use a second-level model to look at
> the pre-post effects.
This feels like good advice, and I'll give it a go. It seems like it's
answering a somewhat different question than the first analysis,
though, no?
Analysis 1 compares the connectivity with a seed region during a
condition from post-training to pre-training, and asks: "Over the
course of training, did the regions connected to this seed during task
performance change?"
Analysis 2 asks: "Is the connectivity between regions different
depending on task condition and, if so, does this difference change
with training?" Or, in other words, "Do we observe a difference of
differences in connectivity?"
That second analysis feels like it's going to be hard to interpret,
even if I do see significant clusters, which is why I was a bit
hesitant to start there.
Again, thanks, and please let me know if I've misinterpreted what the
PPI is actually asking!
Todd
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